I've got a 10 1/2" slick with a 12" plus section width, on a 15x8 1/2 wheel with 4 7/8 bs, but it needs a 3/8 spacer to fit. I've got a 10" slick in a 16x8 1/2 rim that fits better, no spacer needed. Both of those are slightly larger than a 275-60x15 street tire. Rim diameter is immaterial, just look for something around 28" tall, with 12" or less section width on the rims you want. I also have a set of 255-70x15 that barely fit, because of too little backspace.
4 1/2 backspace seems to work best on an 8 1/2 rim, adjust for wider or narrower rim.
My Savoy is a 62 coupe. If I remember right, a wagon has a different shape to the wheel openings, you may be able to get more under the wagon.
Biggest I've used on the front is 235-75x15 on a seven inch rim.